“…The linear DNAs have covalently closed hairpin telomeres (Barbour and Garon, 1987;Hinnebusch et al, 1990;Hinnebusch and Barbour, 1991;Casjens et al, 1997b;Fraser et al, 1997). Most of the plasmids can be lost and are not required for propagation of the bacteria in culture, but loss of infectivity in mice often parallels plasmid loss (for example Barbour, 1988;Hyde and Johnson, 1988;Schwan et al, 1988;Norris et al, 1992;Sadziene et al, 1993b;Persing et al, 1994;Xu et al, 1996;Zhang et al, 1997; but see Casjens et al, 1997a). For brevity, we will refer to the Borrelia extrachromosomal DNA elements as`plasmids', even though some of them may be universally present and are probably essential in nature (see Marconi et al, 1996a;Casjens et al, 1997b;Tilly et al, 1997), carry genes that may be metabolically important (Margolis et al, 1994) or have never been lost in culture.…”